1. Field of the Invention
The present invention relates to a Ti-Al intermetallic compound having an improved room-temperature ductility and high-temperature oxidation resistance, and suitable for use as a high-temperature heat-resistant strength material for aircraft turbine engines, gas turbines for power generators, automobile engines, rotation bodies and the like, and further, to a process for the preparation of this intermetallic compound.
2. Description of the Related Art
The Ti-Al intermetallic compound has almost the highest high-temperature specific strength among metallic materials, and furthermore, has an excellent corrosion resistance and a light weight. Note, it was reported in Metallurgical Transaction, Vol. 6A (1975), page 1991, that a high-temperature strength of 40 kg/mm.sup.2 was obtained at 800.degree. C., and therefore, it is considered that the Ti-Al intermetallic compound is most suitable for application to parts of gas turbines, valves and pistons of automobile engines, high-temperature dies, bearing parts and the like, due to the foregoing excellent characteristics.
The Ti-Al intermetallic compound has a composition latitude in the phase diagram, and in the composition range of 40 to 52 atomic % of Ti and 60 to 48 atomic % of Al, an Llo structure (basically a face-centered tetragonal structure but wherein the Ti layers and Al layers are arranged alternately in the [001] direction) is formed in the thermally equilibriated state. Accordingly, an abnormal strengthening phenomenon wherein the strength is increased in the single crystal state with an increase of the temperature was found, and it is known that, even in the case of polycrystalline materials, the strength is not reduced at a high temperature of up to 800.degree. C. Nevertheless, the polycrystal of the Ti-Al intermetallic compound is defective in that the ductility is low at temperatures ranging from room temperature to about 700.degree. C. For example, in the case of a composition of 48 atomic % of Ti and 52 atomic % of Al, the compressibility is 0.4% at room temperature and about 1.1% at 700.degree. C. (see Japanese Examined Patent Publication No. 59-581).
The difficulties encountered in the development of a Ti-Al intermetallic compound as a practical material are mainly concerned with how to maintain a good room-temperature ductility, and it has been confirmed that an addition of Mn is effective for this purpose (see Japanese Unexamined Patent Publication No. 61-41740). It has been reported, however, that the addition of Mn leads to a lowering of the high-temperature oxidation resistance (Tsurumi et. al., Symposium of Japanese Association of Metals, Plastic Deformation of Ordered Alloys and Intermetallic Compounds, page 13, Jul. 16, 1988).
Further, since the Ti-Al intermetallic compound has a light weight, a high heat-resistance, and an excellent corrosion resistance, it is suitable for a turbine blade to be used at high temperatures. Moreover since the room-temperature ductility of the Ti-Al intermetallic compound is low (the compressibility is 0.4%), a casting or forging thereof is difficult and the safety reliability at room temperature is poor, and thus a practical utilization thereof is uncertain. Moreover, as a practical material for designing, a room-temperature ductility is necessary.